1. Technical Field
The present invention relates to technology for printing images by forming dots on a printing medium.
2. Related Art
Printing devices that perform printing by using print heads while performing scans in main scanning direction and sub scanning direction include inkjet printers such as serial scan type printers, drum scan type printers, and the like. The inkjet printers form characters and images on printing media by ejecting inks from a plurality of nozzles of print heads.
One of dot recording modes employed by the inkjet printers is a mode referred to as “interlace mode”. FIG. 27 is an illustration depicting how sub scan feed is performed in an interlace recording mode. A print head 1000 has four nozzles arranged in sub scanning direction. Numbers 0-3 indicated in circles are nozzle numbers. The nozzles are arranged at a pitch k of three dots in the sub scanning direction. Here, a unit of [dot] means a dot pitch [inch] in the sub scanning direction equivalent to a print resolution. Positions of the print head 1000 indicated as pass 1, pass 2, and so on in FIG. 27 represent positions in the sub scanning direction at the time of each main scan. Here, the “pass” means one main scan. After each main scan, a sub scan feed of a fixed feed amount L of four dots is executed.
However, positions of dots formed by each nozzle may sometimes be misaligned in some degree in the sub scanning direction due to manufacturing error of nozzle. A dot pattern Dtp1 of FIG. 27 is obtained under assumption that no such manufacturing error exists and all dot positions are normal. On the other hand, in case where dots formed by e.g. a first nozzle are misaligned upwards, there may be a space between a main scan line that has dots formed by the first nozzle and a main scan line that has dots formed by a zero nozzle, as shown in a dot pattern Dtp2 of FIG. 27. Such space may be observed by the naked eye as a portion of degraded image quality appearing as a streak, and is referred to as “banding”. The banding may be attributed not only to the manufacturing error of nozzle but also to several factors including error in sub scan feed, warpage of printing paper, and the like.
In order to suppress such banding, a technique that performs correction by biasing density of each raster line is proposed (Patent Document 1). Specifically, in the example of the dot pattern Dtp2 of FIG. 27, the technique makes degradation of image quality appearing as a streak less noticeable by correcting and increasing densities of raster lines 6, 7 as well as by correcting and reducing densities of raster lines 4, 5. In JP-A-2005-219286, a technique that performs a smoothing process of correction value based on densities of a plurality of neighboring raster lines is further proposed.
Conventional techniques are under assumption that density of each raster line is reproduced accurately as long as no error exists in landing position of ink droplet, amount of ink, and the like. However, no consideration has been given to accuracy of density of each raster line in such case of no error. Furthermore, such problem has occurred not only as the problem of banding that is now becoming obvious, but also as a wider range of problem including irregularity of density, fidelity of color reproduction, and the like. In addition, such problem has been occurring not only in inkjet printers but also in laser printers.